Today, Apple released their highly-anticipated iOS 5 update. To a lesser degree of buzz, Foursquare released an update to their app, version 4.0. And it looks like Foursquare v4.0 is going to make good use of that iOS 5 update.

With v4.0 comes Radar, a new feature that basically operates as location-based recommendations and notifications for your mobile device. Before I talk about what Radar does, it’s important to note that Radar is only available right now for iOS devices running the brand new iOS 5.

Foursquare says that “it helps you discover more about the world.” It definitely looks like it could give you more ideas about what to do given your current location. With Radar, Foursquare will notify you when you are in the area of something that’s happening.

For instance, let’s say that I added a new Indian restaurant to my to-do list on the app. Radar now reminds me when I am in the vicinity. It works in the same way with the new Foursquare Lists feature. If I follow a list, for instance “Best Irish Bars in Chicago,” Foursquare Radar will notify me when I am near one of the bars on that list.

Radar will also let me know if my friends are getting together nearby, based on their check-ins. This way I can join up with them if I would like, or maybe stop by and crash the party – if you want to look at it like that.

Once you turn on the new Radar feature, it will begin notifying you about all your best location-based options – the app doesn’t have to be open.

Speaking at a tech event in April, Foursquare CEO Dennis Crowley talked about the future of the company, saying that he wanted things to be more passive than active. Instead of having to open up the app and do all the work of searching your list, and checking out your friends’ activity, Foursquare could suggest things to the user.

This definitely looks like a part of that overall plan. Radar makes it so that the world of Foursquare can exist outside the app.

When you download the new v4.0, you’ll notice another huge change for Foursquare: the ball in the logo is now green. “Nobody freak out,” they say. “Say goodbye to purple balls.”

Gladly.